The going rate for nanny staying overnight?

Myself and DP are hoping to go away for two nights at some point during the week. We have a nanny for our two children (2 and 4) who works 8am to 6.30pm. The kids go to bed between 7pm and 7.30pm, and wake up anytime between 7am to 8am.

A friend of mine was saying she pays an hourly rate on top of their regular hours until the children go to bed / after they wake up, then a set fee for an overnight stay.